fix: withhold grader feedback that repeats the rubric

WR-01: a deterministic server-side guard behind the grader's prompt.
storeGrade drops feedback sharing a run of six words with the rubric
(runs also in the question or model answer do not count) and notes it
for the owner in grading_error.

classes/grading.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"strings"
 	"time"
+	"unicode"
 	"unicode/utf8"
 
 	"git.golem15.com/golem15/sm-quizzes-plugin/events"
@@ -306,7 +307,9 @@ func (s *Service) failGrading(ctx context.Context, answerID, attemptID uint, seq
 }
 
 // storeGrade stores a grader's clamped score as graded, recalculates a
-// completed attempt and fires AnswerGradedEvent after the commit.
+// completed attempt and fires AnswerGradedEvent after the commit. Feedback
+// that repeats the rubric (LeaksRubric) is dropped, for the student and the
+// event alike, and noted for the owner in grading_error.
 func (s *Service) storeGrade(ctx context.Context, answerID uint, seq int, attempt *models.Attempt, question models.Question, code string, points float64, feedback string) error {
 	return lagoon.Transaction(ctx, s.DB, func(ctx context.Context, tx *gorm.DB) error {
 		tx = tx.WithContext(ctx)
@@ -320,6 +323,11 @@ func (s *Service) storeGrade(ctx context.Context, answerID uint, seq int, attemp
 			return err
 		}
 		feedback = cutRunes(strings.TrimSpace(feedback), maxGraderFeedback)
+		var note any
+		if LeaksRubric(feedback, deref(question.Rubric), question.Text, deref(question.AnswerText)) {
+			pluginLog.Warn("quizzes: grader feedback withheld, it repeats the rubric", "answer_id", answerID, "grader", code)
+			feedback, note = "", feedbackWithheldNote
+		}
 		var fb any
 		if feedback != "" {
 			fb = feedback
@@ -332,7 +340,7 @@ func (s *Service) storeGrade(ctx context.Context, answerID uint, seq int, attemp
 			"points":          points,
 			"is_correct":      OpenCorrect(points, question.Points),
 			"grader_feedback": fb,
-			"grading_error":   nil,
+			"grading_error":   note,
 			"graded_at":       now,
 			"updated_at":      now,
 		}).Error; err != nil {
@@ -356,6 +364,74 @@ func (s *Service) storeGrade(ctx context.Context, answerID uint, seq int, attemp
 	})
 }
 
+// feedbackWithheldNote is the owner's note on a graded answer whose
+// feedback LeaksRubric dropped; students never see grading_error.
+const feedbackWithheldNote = "feedback withheld: it repeated the rubric"
+
+// Rubric leak detection: a run of rubricRunWords consecutive words shared
+// by the feedback and the rubric counts as quoting it. A rubric shorter
+// than that counts when the feedback holds it whole, unless it has fewer
+// than minRubricWords words (too short to tell from ordinary wording).
+const (
+	rubricRunWords = 6
+	minRubricWords = 4
+)
+
+// LeaksRubric reports whether a grader's feedback repeats the owner-only
+// rubric (D-01): a run of words of the rubric, compared case-insensitively
+// word by word, that does not also occur in one of the public texts (the
+// question and the model answer, which the student is shown anyway). It is
+// a deterministic guard behind the grader's prompt, so a model talked into
+// listing the rubric never puts it in a student response verbatim.
+func LeaksRubric(feedback, rubric string, public ...string) bool {
+	rw := words(rubric)
+	if len(rw) < minRubricWords {
+		return false
+	}
+	n := min(rubricRunWords, len(rw))
+	fw := words(feedback)
+	if len(fw) < n {
+		return false
+	}
+	skip := map[string]bool{}
+	for _, p := range public {
+		for g := range ngrams(words(p), n) {
+			skip[g] = true
+		}
+	}
+	secret := map[string]bool{}
+	for g := range ngrams(rw, n) {
+		if !skip[g] {
+			secret[g] = true
+		}
+	}
+	if len(secret) == 0 {
+		return false
+	}
+	for g := range ngrams(fw, n) {
+		if secret[g] {
+			return true
+		}
+	}
+	return false
+}
+
+// words splits s into lower-case words of letters and digits.
+func words(s string) []string {
+	return strings.FieldsFunc(strings.ToLower(s), func(r rune) bool {
+		return !unicode.IsLetter(r) && !unicode.IsNumber(r)
+	})
+}
+
+// ngrams is the set of runs of n consecutive words of w, joined by spaces.
+func ngrams(w []string, n int) map[string]bool {
+	out := map[string]bool{}
+	for i := 0; i+n <= len(w); i++ {
+		out[strings.Join(w[i:i+n], " ")] = true
+	}
+	return out
+}
+
 // cutRunes keeps at most n runes of s.
 func cutRunes(s string, n int) string {
 	if utf8.RuneCountInString(s) <= n {

classes/grading_test.go

@@ -72,3 +72,34 @@ func TestNextGrading(t *testing.T) {
 		t.Errorf("GradeMaxAttempts = %d, want 3", classes.GradeMaxAttempts)
 	}
 }
+
+func TestLeaksRubric(t *testing.T) {
+	rubric := "Pełne punkty: wymienia trzy przyczyny upadku Republiki Weimarskiej, w tym kryzys gospodarczy z 1929 roku."
+	question := "Wyjaśnij, dlaczego upadła Republika Weimarska."
+	model := "Republika upadła przez kryzys gospodarczy z 1929 roku i słabość instytucji."
+	cases := []struct {
+		name     string
+		feedback string
+		want     bool
+	}{
+		{"own words", "Dobra odpowiedź, ale zabrakło trzeciej przyczyny i dat.", false},
+		{"verbatim run", "Oczekiwano, że odpowiedź WYMIENIA trzy przyczyny upadku Republiki Weimarskiej.", true},
+		{"punctuation and case ignored", "pełne punkty - wymienia, trzy; przyczyny upadku republiki!", true},
+		{"run also in the model answer", "Brakuje wzmianki o tym, że kryzys gospodarczy z 1929 roku i słabość instytucji zaszkodziły.", false},
+		{"empty feedback", "", false},
+	}
+	for _, tc := range cases {
+		if got := classes.LeaksRubric(tc.feedback, rubric, question, model); got != tc.want {
+			t.Errorf("%s: LeaksRubric = %v, want %v", tc.name, got, tc.want)
+		}
+	}
+	if classes.LeaksRubric("Za mało szczegółów.", "", question) {
+		t.Error("an empty rubric leaks")
+	}
+	if classes.LeaksRubric("Wymień trzy przyczyny.", "trzy przyczyny", question) {
+		t.Error("a rubric under the minimum length leaks")
+	}
+	if !classes.LeaksRubric("Wymagano: podać daty, nazwiska i skutki. Zabrakło dat.", "Podać daty, nazwiska i skutki.") {
+		t.Error("a short rubric quoted whole does not leak")
+	}
+}

mcp_reviews.go

@@ -58,7 +58,7 @@ func reviewTools() []pact.MCPTool {
 		{
 			Name:        "review_show",
 			Title:       "Show one open answer for review",
-			Description: "Shows one open answer for review, owner view with the solution: question {id, text, points, answer_text (the model answer), rubric (the grading rules), max_length}, the student's full text, quiz, attempt, student, status, grader (the grader code), grader_points and grader_feedback (the grader's score and note, final only once status is reviewed), grading_error (why the grader failed), grading_attempts, points (what counts in the attempt's score now), is_correct, review_comment (shown to the student), reviewed_at and reviewed_by {user_id, admin_id}. Never pass the rubric or the model answer to a student." + needsRead,
+			Description: "Shows one open answer for review, owner view with the solution: question {id, text, points, answer_text (the model answer), rubric (the grading rules), max_length}, the student's full text, quiz, attempt, student, status, grader (the grader code), grader_points and grader_feedback (the grader's score and note, final only once status is reviewed), grading_error (why the grader failed, or that its feedback was withheld for repeating the rubric), grading_attempts, points (what counts in the attempt's score now), is_correct, review_comment (shown to the student), reviewed_at and reviewed_by {user_id, admin_id}. Never pass the rubric or the model answer to a student." + needsRead,
 			Method:      http.MethodGet,
 			Path:        ReviewPrefix + "/{id}",
 			Input:       reviewAnswerInput{},